This invention relates to an antenna diversity receiver for selecting and coupling one of a plurality of antennas to a RF receiver input with a multipath detector to activate changing the antenna at the occurrence of multipath interferences in the RF reception signal and automatically disabling the switching during a disabling period following each such change over.
The reception of a wanted RF broadcast transmitter signal may be disturbed or otherwise deteriorated by various phenomena, such as multipath reception and/or adjacent channel interferences. In general, multipath reception is caused by signal reflections at and/or against environmental physical obstacles such as mountains, trees, buildings, fences and the like. Due to such signal reflections an RF broadcast signal may arrive at a certain reception location through various different signal paths in various different amplitude and phase conditions. The summation of these multipath signals at the antenna of the receiver results in unpredictable signal amplitude and/or phase distortions. These effects often result in partial or complete cancellation of the useful RF reception signal. These signal cancellations, hereinafter also being referred to as signal dips, strongly depend on the RF carrier frequency of the received RF broadcasting signal and on the location of reception. The signal dips severely deteriorate the wanted RF broadcasting signal and therefore the overall signal reception quality as well.
However, a relatively small shift in the position of the antenna suffices to strongly improve the signal reception quality. This solution is used in antenna diversity receivers to avoid reception of multipath distorted RF signals and in particular with mobile FM receivers. Such antenna diversity receivers are provided with two or more mutually spaced apart antennas which may be coupled to a RF input of a receiver. Only the antenna having best local receiving conditions with respect to the other antenna(s) is actually connected to the RF receiver input. This antenna, also referred to as the actual antenna, is effective in the reception and supply of the wanted RF broadcasting signal to the receiver as long as the multipath distortion at the actual antenna remains smaller than a certain predetermined multipath threshold level. As soon as the received multipath distortion exceeds the certain predetermined multipath threshold level, a change over in the RF signal supply to the receiver from this actual antenna to another antenna positioned at a location with better receiving conditions, is initiated. The receiver is thus continuously optimized for minimum multipath reception.
Inherent to antenna diversity receivers are short interruptions in the RF signal supply to the receiver during an antenna change over or switching action. Due to the delay between the occurrence of an actual multipath caused signal dip and the detection of the dip, the RF signal interruptions may be detected as being caused by multipath effects, and may therewith initiate a subsequent false antenna switching action. Such false antenna switching may in its turn be detected as a multipath originated signal dip again, initiating a further false antenna switching action, and so on and so forth, with an oscillating effect as a result. To reduce the risk of oscillating antenna switching, antenna diversity receivers which use the Philips"" IC TEA 6101 antenna diversity integrated circuit may be provided with a switching disabling circuit which automatically disables the antenna selective switching circuit to change antennas over a certain predetermined fixed disabling period following each such change over or switching action following a preceding switching action within a certain predetermined fixed time period.
This method is not effective in receivers with dynamic IF selectivity, such as receivers with adjacent channel suppression. As explained above, adjacent channel interferences are another important source of signal distortion and are usually caused by FM radio broadcast signals modulated on a carrier positioned infrequency adjacent to the carrier frequency of a wanted FM radio broadcast signal. Due to peak values in the FM modulation signal, these adjacent channel FM radio signals may temporarily exceed the allocated channel bandwidth and break through into the frequency range of the wanted FM radio broadcast signal. In receivers with adjacent channel suppression such adjacent channel interferences are suppressed by varying the bandwidth of the variable bandwidth intermediate frequency (IF) selective circuit. This is dependent on the deviation of the adjacent channel signal within the frequency range of the wanted FM radio broadcast signal. The larger the deviation, the smaller the bandwidth of the variable bandwidth intermediate frequency (IF) selective circuit. The dynamically varying IF selectivity thus effectuates a suppression of the adjacent channel interferences. However, this method cannot capture the benefits of using diverse antennas.
There is thus a need for a receiver which combines the benefits of the antenna diversity feature with those of the dynamic IF selectivity feature. There is a further need for a receiver which combines antenna diversity with dynamic IF selectivity while preventing unwanted effects. There is also a need for a receiver which to maximizes the area in which the antenna diversity feature is operative, without giving rise to switching oscillating effects.
The needs may be met by the present invention which may be embodied in an antenna diversity receiver having antenna selective switching means for selecting and coupling one of a plurality of antennas to an RF receiver input. The receiver includes a multipath detector to activate the antenna selective switching means to change over to another antenna at the occurrence of multipath interferences in the RF reception signal of the one antenna. A switching disabling means automatically disables the antenna selective switching means to change antennas during a disabling period following each such change over. An adjacent channel detector controls the disabling period to increase with increasing adjacent channel interferences and vice versa.
The receiver is based on the recognition that the delay between the actual occurrence of a multipath caused signal dip and its detection is determined by the receiver""s IF selectivity in an antenna diversity receiver. In a receiver with dynamic IF selectivity, the IF bandwidth is variable, causing the delay to vary as well. The smaller the IF bandwidth the longer the delay and vice versa. However, the longer the delay, the longer the time period between the switching actions required to avoid the risk of oscillating false antenna switching actions.
The present invention combines the features of antenna diversity and dynamic IF selectivity, while preventing antenna switching actions from oscillating. The invention also allows accurate definition when to disable such antenna switching actions. The disabling period is dynamically varied with the delay between the actual occurrence of a multipath caused signal interference and the detection of the interference with the IF bandwidth. This is in contrast to the above mentioned receiver which uses a fixed disabling period, during which switching actions are disabled irrespective of the actual delay, (the actual IF bandwidth). This may cause unnecessary disabling of antenna switching actions, and in particular this will occur at greater IF bandwidth. By applying the present invention, such unnecessary disabling of antenna switching actions may be prevented, allowing the area in which the antenna diversity feature is operative to be maximized without giving rise to switching oscillating effects.
For reliable implementation, the antenna diversity receiver is preferably characterized by the adjacent channel detector being coupled to a control terminal of a variable timer circuit defining the disabling period. The variable timer circuit may include a digital counter having a variable counting cycle. Detecting switching actions is preferably implemented by an antenna switching detection means with a signal level detection means and an OR gate coupled between said plurality of antennas and the switching disabling means. A cost-effective embodiment of the invention is characterized by the OR gate means having an exclusive OR gate for each pair of antennas. The antennas within each such pair are coupled through the signal level detection means to the first and second input terminals of the exclusive OR gate, directly and via a delay element. An output of each of the exclusive OR gates is coupled to an input of a nonexclusive OR gate which has an output coupled to the switching disabling means.
A further cost reduction is obtained with a delay element having an RC member with a serial resistor and a mass connected capacitor. The joint connection of the resistor and the capacitor are coupled to the signal level detection means by respectively, the signal level detection means coupled to the second input terminal of the exclusive OR gate.
The antenna diversity receiver according to the invention is characterized by a threshold circuit coupled between an output of the adjacent channel detector and a control input of the switching disabling means to accurately match the control range of the variable timer circuit to the control range of the IF bandwidth. The switching disabling means compares the output signal of the adjacent channel detector with a threshold value limiting the control range of the variable timer circuit at a predetermined value.
This measure also allows for combining the antenna disabling feature based on a fixed disabling period used in the known receiver with the disabling feature according to the invention. The adjacent channel detector may be used for the antenna disabling feature as well as for controlling the bandwidth of a variable bandwidth intermediate frequency (IF) selectivity to decrease bandwidth with increasing adjacent channel interferences and vice versa, therewith realizing the feature of dynamic selectivity or adjacent channel suppression.
In yet another preferred embodiment, the plurality of antennas includes an antenna for receiving radio broadcast RF signals as well as an antenna for receiving telecommunication RF signals.
It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention.